This invention relates to joints produced by the self-piercing riveting (SPR) process. More particularly, it relates to maximizing the strength of an SPR lap joint that consists of two rows of self-piercing rivets without increasing the width of the overlap region (flange width) or the quantity of material used in any manner.
In self-piercing riveting (SPR), a tubular rivet made from a high-strength steel alloy is forced through a pair of partially overlapping sheets that are supported by a rigid circular die with an axisymmetric cavity. The diameter of the die and rivet are similar. The sheet material is typically an automotive aluminum alloy such as AA6111-T4 or AA5754-O. The joint is cold-formed with the rivet walls experiencing large amounts of compressive plastic deformation. The upper sheet is pierced through its entire thickness by the rivet, predominantly in shear, and the lower sheet is pierced only partially. Piercing forces cause the lower sheet to flow into the die cavity locally and conform to the cavity shape. The entire process is completed in about one second.
Any means of increasing the mechanical (static and fatigue) strengths of SPR joints through the modification of only particular values of process parameters within the existing set is highly desirable. Current methods for producing SPR joints generally involve riveting from one direction only. Even when practical constraints force riveting from opposite directions, the influence of different rivet orientations on joint mechanical (static and fatigue) strength is unknown.
The present invention relates to the maximization of joint strength (static and fatigue) by selecting a particular combination of riveting directions, i.e., rivet orientations. It also provides design-guidelines relating to the variation in joint mechanical strength for different combinations of riveting orientation.
The present invention provides an assembly and method, which increases the mechanical (static and fatigue) strength of a self-piercing riveted lap joint without increasing flange width, that is, material used in the overlap region. Strength is increased by inserting pairs of adjacent rivets on opposite sides of the flange in a particular configuration. The rivets are driven into the opposite sides of the flange by using multiple rivet driving apparatuses, which are capable of driving rivets in opposite directions.
When a tension load is put upon a riveted flanged assembly, the area surrounding a rivet becomes a high stress area. Testing shows the area around the head of the rivet tends to be the area of highest stress. As a result, the high stress area around the head of the rivet tends to break before any other part of the assembly. It has been determined that the mechanical strength of a joint can be maximized by placing the heads of rivets on opposing sides of a flanged assembly near the interior portions of the flanges, closest to the loaded ends of the sheets.
By dividing the number of high stress areas between both flanges of the assembly, the stress is shared between the metal sheets. As a result, the strength of the joint holding the assembly together is increased without increasing the width of the flange.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.